1. Field of the Invention
The present invention relates generally to a driving circuit for a flat panel display device and, more particularly, to a driving method and circuit for the automatic voltage output of an active matrix organic light emitting device, which is capable of resolving the non-uniformity of brightness between pixels, that is, a great problem in a flat panel display using the active-matrix organic light emitting device, and a data drive circuit using the same.
2. Description of the Related Art
Technologies for forming a Thin Film Transistor (TFT) on a substrate have been widely developed for the past several years, and applications of the technologies to an active-matrix display device are being developed.
Particularly, since a TFT using a poly-silicon film has field-effect mobility higher than that of a conventional TFT using an amorphous silicon film, the TFT using the poly-silicon film can operate at a high speed.
As a result, pixel control, which was conventionally performed by a drive circuit located outside a substrate, can be performed by a drive circuit formed on the same substrate as pixels.
Such a type of active-matrix display device has been focused on because of its many advantages, such as low manufacturing cost, the miniaturization of the display device, high yield, and high throughput, that can all be acquired by integrating various circuits and devices on the same substrate.
Currently, active-matrix Electro-Luminescent (EL) display devices having EL devices as self-light-emitting devices are actively being studied. An EL display device is also called an Organic EL Display (OELD) or an Organic Light Emitting Device (OLED), while an Active-Matrix Organic Light Emitting Device is called an AMOLED.
Unlike liquid crystal display devices, an organic display device is a self-emitting type. An EL device is constructed such that EL layers are interposed between a pair of electrodes. When electrons and holes are respectively injected to the EL layers, which are formed between a first electrode (negative), that is, an electron-injection electrode (cathode), and a second electrode (positive), that is, a hole-injection electrode (anode), the electrons and the holes are respectively combined to form electron-hole pairs and then create excitons. The created excitons disappear while transitioning from a excited state to a ground state, thereby emitting light.
Such an OLED operates with a bias of 2 to 30 volts. The brightness of the OLED can be controlled by adjusting voltage or current applied to the anode and cathode thereof. The relative amount of generated light is called a gray level. In general, the OLED optimally operates in current mode.
Light output is more stabilized upon constant-current driving rather than upon constant-voltage driving. This stands a contrast to the operation of many other displays that operate in voltage mode. As a result, an active matrix display using the OLED technology requires a specific pixel structure in order to provide current operation mode.
Generally, in a matrix address OLED device, a plurality of OLEDs is formed on a single substrate and is arranged in regular grid pattern groups. The several OLED groups, which form a grid column, can share a common cathode or a cathode line. The several OLED groups, which form a grid row, can share a common anode or an anode line.
Respective OLEDs of a predetermined group emit light when the cathode and anode lines thereof are simultaneously activated. Each OLED group of a matrix may form one pixel of a display, and each OLED acts as a sub-pixel or a pixel cell.
An OLED has excellent characteristics, such as a wide viewing angle, fast response and high contrast, so that it may be used as a pixel of a graphic display, a television video display or a surface light source. Furthermore, the OLED can be formed on a flexible transparent substrate, such as plastic, can be formed to be thin and light, and has good color sensitivity, so that it is suitable for a next-generation flat panel display.
Furthermore, the OLED can represent 3 colors, that is, red (R), green (G) and blue (B), and does not require backlight, as opposed to the well-known Liquid Crystal Display (LCD), thereby decreasing power consumption. The OLED has good color sensitivity, so that it attracts attention as a full-color display.
FIG. 1 is a schematic block diagram illustrating a conventional data drive circuit disclosed in U.S. Pat. No. 6,795,045. The conventional data drive circuit includes a current drive unit 12 for supplying constant current to a panel 11, a light-emitting time detecting unit 13 for detecting light-emitting time and a digital signal processing unit 14 for controlling the light-emitting time.
Furthermore, as illustrated in FIG. 2, a conventional basic pixel circuit (disclosed in U.S. Pat. No. 5,684,365) includes a drive transistor M1, a data line, a switch transistor M2, a data storing capacitor C and an OLED.
In the conventional data drive circuit of FIG. 1, a method of detecting voltages generated in respective pixels by drive current, comparing the detected voltages with a reference voltage, and defining light-emitting time based on comparison results in order to detect the light-emitting time, was proposed.
However, it is well known that voltage-current characteristics of drive transistors, which constitute respective pixels, differ from each other. This implies that, when different pixels are driven with the same current, voltages, which are induced in the drive transistors of respective pixels, differ from each other. Furthermore, it is impossible that the induced voltages are used as the brightness information of the pixels unless they are used upon digital driving.
Furthermore, in the AMOLED pixel circuit of FIG. 2, a basic principal of data driving is to control brightness using the amount of current which flows through the OLED. As in FIG. 1, the determination of the brightness of an OLED based on the detection of the voltage of a data line is possible only when the voltage-current characteristics of drive transistors, which constitute respective pixels, are identical to each other. In fact, the voltage-current characteristics of drive transistors are different.
As a result, in order to control the amounts of current, which flows through the OLEDs of respective pixels, to be uniform, it is necessary to directly monitor and control current on data lines.
That is, a principal cause of the non-uniformity of brightness (such as panel-to-panel non-uniformity, and pixel-to-pixel non-uniformity in a panel) in a flat panel display using AMOLEDs is that the drive transistors of respective pixels, constituting the flat panel display, have different characteristics depending on pixels and the characteristics randomly vary over time.
As a result, a solution to the non-uniformity of the drive transistors constituting respective pixels has been sought to achieve the uniformity of the brightness of a flat panel display.